1. Field of the Invention
The present invention is related to methods of applying a liquid to a substantially vertical surface. In particular, this invention is related to methods of applying liquids to substantially vertical surfaces to form substantially non-sag films or coatings.
2. Related Background Art
Liquid coatings applied to vertical surfaces are susceptible to defects commonly known as sagging, running, dripping and curtaining, as described in Percy E. Pierce and Clifford K. Schoff, Coating Film Defects (Federation of Societies for Coatings Technology, 1988), the disclosure of which is incorporated by reference herein. These defects arise when the action of gravitational forces on the coating film results in downward flow of the film.
It is often desirable to form a film or coating, that is substantially without the above-described defects, on a substrate surface. Such non-sag films provide consistent and isotropic properties throughout the film or coating. Properties such as color, opacity, resistance, and mass are typically desired to be uniform in coatings or films. Non-uniformity of thickness caused by sagging, dripping curtaining or running can lead to undesirable color gradients, uneven hiding of the substrate hue, areas more prone to corrosion, and "orange peeling".
In particular, it is often desirable to form a film or coating that is substantially even, without sag, on a substrate that is substantially inclined, especially a substrate at or very near vertical.
The conventional remedies involve modification of the viscosity or thickness of the film. Reducing the film thickness decreases the flow velocity in the liquid coating, allowing the coating to dry or cure before noticeable defects arise. However, the decreased film thickness requires multiple applications of coating to achieve thicker overall coating thicknesses. Increasing the coating viscosity also decreases the flow velocity, and is typically accomplished by adding thickeners or thixotropes, or by using a solvent that evaporates relatively quickly. Modifying the coating formulation in this way, however, adds to the cost of the coating and may be detrimental to other properties of the coating.
A particularly difficult problem arises when a non-sag coating is required from liquids with low or zero solids loadings. Such mixtures of miscible liquids can undergo wide changes in rheological properties as their component ratios change because of the different evaporation rates of each component. This problem, of how to form a non-sag coat of miscible liquids to a substantially vertical surface, has not been adequately addressed.
U.S. Pat. No. 4,018,953 describes mounting, on a cylinder, a coating collar that forms a trough into which is poured the coating liquid. The liquid is applied to the cylinder by sliding the collar down the cylinder to form a thin coating of the liquid on the outer surface of the cylinder.
U.S. Pat. Nos. 4,345,546 and 4,455,322 describe methods and apparatus for coating items in which an object is immersed into a solution and removed from the solution at a constantly changing speed in order to form a coating that is thinner at the top and bottom and thicker at the center.
U.S. Pat. No. 4,597,931 describes forming a windshield with a varying thickness produced by dipping the windshield base in a coating solution and withdrawing the base from the solution at a varying speed.
U.S. Pat. No. 5,002,809 describes applying an urethane base coating material to a vehicle surface in overlapping coatings.
U.S. Pat. No. 5,063,085 describes a coating method that applies a coating to an object and rotating the object to prevent sagging.
U.S. Pat. No. 5,153,027 describes applying a varying thickness coating by immersing an object into a bath of the coating liquid and withdrawing the object in a predetermined direction at a predetermined rate while simultaneously pivoting the object at a predetermined angular velocity.
U.S. Pat. No. 5,212,000 describes applying nonNewtonian coating fluids to the inner surfaces of hollow tubes.
U.S. Pat. No. 5,521,477 describes a method for evaluating coating sag in order to teach a robotic spray coater to apply a uniform coat. The patent mentions surface tension without providing any guidance about how to use surface tension to control the uniformity of an applied coating.
U.S. Pat. No. 3,782,328 describes spray coating a horizontal substrate with a squeegee/shield apparatus downstream of the spray in order to prevent formation of non-uniform banding or streaking.
U.S. Pat. No. 3,994,701 describes a method of applying a uniform thickness of a processing fluid by tapering the spread-thickness control components of the film assemblage in order to provide more resistance to movement during the beginning of the process of spreading the fluid, when there is more fluid to be spread, and less resistance later as the amount of the fluid to be spread decreases.
U.S. Pat. No. 4,138,284 describes a method to form a colored shade band on a substrate by spray coating along a mask edge.
U.S. Pat. No. 4,152,807 describes a scrubbing attachment to apply a solution to a window.
U.S. Pat. No. 4,527,507 describes spray apparatus with overlapping spray patterns that have overlap-coat regions as thick as the center single-coat regions.
U.S. Pat. No. 5,141,156 describes obtaining a feathered spray of a liquid using airless spray techniques.
U.S. Pat. No. 5,500,274 describes providing a laminated glazing having an optical density gradient band by forming a coating of variable thickness and gradient coloration in the cross-web direction.
U.S. Pat. No. 5,705,470 describes a sprayable cleaning gel composition that clings to a surface without running.
A method for producing a substantially non-sag coating of miscible liquids on a vertical surface would be extremely useful. Such a method would be very useful in many applications. Such applications include, for example, window cleaners, oven cleaners, and any application that would benefit from a non-sag coating of miscible liquids being formed on a vertical surface.